LMFP cathode materials with improved electrochemical performance

ABSTRACT

Particulate LMFP cathode materials having high manganese contents and small amounts of dopant metals are disclosed. These cathode materials are made by milling a mixture of precursor materials in a wet or dry milling process. Preferably, off-stoichiometric amounts of starting materials are used to make the cathode materials. Unlike other high manganese LMFP materials, these cathode materials provide high specific capacities, very good cycle life and high energies even at high discharge rates.

CROSS-REFERENCE TO RELATED APPLICATIONS

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BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to olivine lithium manganese iron phosphate cathode materials for lithium batteries and to methods for making such materials.

Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98

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Lithium batteries are widely used as primary and secondary batteries for vehicles and many types of electronic equipment. These batteries often have high energy and power densities.

LiFePO₄ is known as a low cost material that is thermally stable and has low toxicity. It can also demonstrate very high rate capability (high power density) when made with a small particle size and a good carbon coating. For these reasons, LiFePO₄ has found use as a cathode material in lithium batteries. However, LiFePO₄ has a relatively low working voltage (3.4V vs. Li+/Li) and because of this has a low energy density relative to oxide cathode materials. In principle, the working voltage and therefore the energy density can be increased by substituting manganese for some or all of the iron to produce a lithium manganese iron phosphate (Li_(a)Mn_(b)Fe_((1-b))PO₄ (LMFP)) cathode, without a significant sacrifice of power capability. However, structural stability and charge transport are adversely affected by replacing iron with manganese, and the specific capacities obtained have fallen significantly short of theoretical levels. Energy and power densities also are disappointingly low. In addition, battery cycling performance for LMFP electrodes often is less than desirable, due to a loss of capacity with cycling.

Attempts have been made to improve the performance of olivine lithium transition metal phosphate electrode materials by adjustments to the stoichiometry, i.e., the ratios of lithium, manganese, iron and the phosphate ions. Electrode materials containing an excess of lithium have been described, for example, in WO 2009/144600 and U.S. Pat. No. 7,842,420. Adding certain “dopants”, i.e., metals other than lithium, manganese and iron, also has been suggested in LMFP electrode materials having low manganese contents. See, e.g., WO 2011/025823, which mentions cobalt, nickel, niobium and vanadium as the “dopant” materials in an LMFP material in which the value of b is up to 0.6. See also U.S. Pat. No. 8,168,150, in which various other metals are suggested in an LMFP cathode material containing a large amount of iron.

There remains a desire to provide an olivine LMFP cathode material that has good specific capacity, high rate performance, and improved cycling performance.

BRIEF SUMMARY OF THE INVENTION

This invention is in one aspect a cathode material comprising an electroactive material having the empirical formula Li_(a)Mn_(b)Fe_(c)D_(d)PO₄, wherein

a is a number from 0.85 to 1.15;

b is from 0.65 to 0.95;

c is from 0.049 to 0.349;

d is from 0.001 to 0.1;

2.75≦(a+2b+2c+dV)≦3.10, wherein V is the valence of D, and D is a metal ion selected from one or more of magnesium, calcium, strontium, cobalt, titanium, zirconium, molybdenum, vanadium, niobium, nickel, scandium, chromium, copper, zinc, beryllium, lanthanum and aluminum, and further wherein at least a portion of the electroactive material has an olivine structure.

The invention is also a nanocomposite containing at least 70% by weight of a particulate electroactive material of the invention with up to 30% by weight of graphite, carbon black and/or other conductive carbon.

The LMFP cathode materials of the invention provide enhanced cycle life while retaining good specific capacity and good high rate performance.

In another aspect, the invention is a method for making an olivine lithium manganese iron phosphate cathode material, comprising

a) forming a mixture of at least one lithium precursor, at least one iron precursor, at least one manganese precursor, at least one dopant metal precursor and at least one precursor of H_(x)PO₄ ions where x is 0, 1 or 2, wherein the precursors are present in amounts such that:

the mole ratio of lithium ions to H_(x)PO₄ ions is 0.85 to 1.15;

the mole ratio of manganese ions to H_(x)PO₄ ions is 0.65 to 0.95;

the mole ratio of iron ions to H_(x)PO₄ ions is 0.049 to 0.349;

the mole ratio of dopant metal ions to H_(x)PO₄ ions is 0.001 to 0.10; and

the mole ratio of lithium, manganese, iron and dopant metal ions combined to H_(x)PO₄ ions is such that (2.75×Moles H_(x)PO₄)≦[moles Li+(2×Moles Mn)+(2×Moles Fe)+(2×Moles Dopant Metal)]≦(3.10×Moles H_(x)PO₄);

b) milling the mixture and then;

c) calcining the milled mixture to form the olivine lithium manganese iron phosphate cathode material.

Cathode materials made in accordance with the invention offer several important advantages over conventional high-manganese LMFP cathode materials. One important advantage is cycle life. The cathode materials formed in accordance with the invention have been found to exhibit excellent cycle lives. Lithium batteries containing cathodes made from these materials tend to retain specific capacity very well over many charge/discharge cycles. Importantly, this high specific capacity often is seen even at high discharge/charge rates. A second important advantage is that those batteries often exhibit high energies, due to a combination of high capacities and high operating voltages. Another significant advantage is that batteries containing these electrodes are capable of high charge rates.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE INVENTION

The FIGURE is a graph showing the specific capacity versus number of cycles for several embodiments of the invention and for several comparative samples.

DETAILED DESCRIPTION OF THE INVENTION

Cathode materials of the invention preferably are made in a milling process, in which the various precursor materials described herein are milled together, followed by a calcining step. The milling step intimately mixes the precursors and in most cases reduces their particle size. Some reaction of the precursors may occur during the milling step, although it is believed that the final olivine structure does not fully develop until the calcining step is performed. Dry or wet milling methods can be used, as described more fully below.

The process of the invention is characterized in part by the ratios of the starting materials. The amounts of starting materials are conveniently expressed in terms of mole ratios of the metal ions (lithium, manganese, iron and dopant metal(s)) to H_(x)PO₄ ions.

The amount of lithium precursor provided is such that the mole ratio of lithium ions to H_(x)PO₄ ions is 0.85 to 1.15. This ratio preferably is 0.95 to 1.10, more preferably 1.0 to 1.10, still more preferably 1.025 to 1.10 and in some embodiments may be 1.025 to 1.075.

The amount of manganese precursor provided is such that the mole ratio of manganese ions to H_(x)PO₄ ions is 0.65 to 0.95. This ratio preferably is 0.70 to 0.95, more preferably 0.70 to 0.85, and in some embodiments may be 0.75 to 0.85.

The amount of iron precursor provided is such that the mole ratio of iron ions to H_(x)PO₄ ions is 0.049 to 0.349. This ratio preferably is 0.10 to 0.30 and in some embodiments may be 0.15 to 0.25.

The amount of dopant metal precursor provided is such that the mole ratio of dopant metal ions to H_(x)PO₄ ions is 0.001 to 0.10. This ratio preferably is 0.005 to 0.10, more preferably 0.01 to 0.075 and in some embodiments may be 0.02 to 0.06.

In addition, the amount of lithium, manganese, iron and dopant metal precursors are chosen together. The mole ratio of lithium, manganese, iron and dopant metal ions combined to H_(x)PO₄ ions is such that (2.75×Moles H_(x)PO₄)≦[moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)]≦(3.10×Moles H_(x)PO₄). The value of [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)] preferably is not equal to 3.00. In some embodiments, [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)] is) 2.85 to 2.99×Moles H_(x)PO₄) or (3.01 to 3.05×Moles H_(x)PO₄). In particular embodiments, [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)] is (2.95 to 2.98×Moles H_(x)PO₄) or (3.01 to 3.05×Moles H_(x)PO₄).

In specific embodiments:

1) the mole ratio of lithium ions to H_(x)PO₄ ions is 0.95 to 1.10, the mole ratio of manganese ions to H_(x)PO₄ ions is 0.70 to 0.95; the mole ratio of iron ions to H_(x)PO₄ ions is 0.049 to 0.349, the mole ratio of dopant metal ions to H_(x)PO₄ ions is 0.005 to 0.10 and [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)] is (2.85 to 2.99×Moles H_(x)PO₄) or (3.01 to 3.05×Moles H_(x)PO₄);

2) the mole ratio of lithium ions to H_(x)PO₄ ions is 1.00 to 1.10, the mole ratio of manganese ions to H_(x)PO₄ ions is 0.70 to 0.95; the mole ratio of iron ions to H_(x)PO₄ ions is 0.1 to 0.3, the mole ratio of dopant metal ions to H_(x)PO₄ ions is 0.005 to 0.10 and [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)] is (2.85 to 2.99×Moles H_(x)PO₄) or (3.01 to 3.05×Moles H_(x)PO₄);

3) the mole ratio of lithium ions to H_(x)PO₄ ions is 1.025 to 1.10, the mole ratio of manganese ions to H_(x)PO₄ ions is 0.70 to 0.85; the mole ratio of iron ions to H_(x)PO₄ ions is 0.1 to 0.3, the mole ratio of dopant metal ions to H_(x)PO₄ ions is 0.01 to 0.075 and [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)] is (2.85 to 2.99×Moles H_(x)PO₄) or (3.01 to 3.05×Moles H_(x)PO₄);

4) the mole ratio of lithium ions to H_(x)PO₄ ions is 1.025 to 1.075, the mole ratio of manganese ions to H_(x)PO₄ ions is 0.75 to 0.85; the mole ratio of iron ions to H_(x)PO₄ ions is 0.15 to 0.25, the mole ratio of dopant metal ions to H_(x)PO₄ ions is 0.02 to 0.06 and [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)] is (2.95 to 2.98×Moles H_(x)PO₄) or (3.01 to 3.05×Moles H_(x)PO₄).

In any of the foregoing embodiments the dopant metal is selected from one or more of magnesium, calcium, strontium, cobalt, titanium, zirconium, molybdenum, vanadium, niobium, nickel, scandium, chromium, copper, zinc, beryllium, lanthanum and aluminum. The dopant metal is preferably magnesium, cobalt, titanium, vanadium, nickel, or aluminum or a mixture of two or more thereof. The dopant metal is more preferably magnesium or a mixture of magnesium and with one or more of calcium, strontium, cobalt, titanium, zirconium, molybdenum, vanadium, niobium, nickel, scandium, chromium, copper, zinc, beryllium, lanthanum and aluminum. The dopant metal is most preferably magnesium.

The precursor materials are compounds other than a LMFP, and are compounds which react to form a LMFP as described herein. Some or all of the precursor materials may be sources for two or more of the necessary starting materials. Suitable lithium precursors include, for example, lithium hydroxide, lithium oxide, lithium carbonate, lithium dihydrogen phosphate, lithium hydrogen phosphate and lithium phosphate. Lithium dihydrogen phosphate, lithium hydrogen phosphate and lithium phosphate all function as a source for both lithium ions and H_(x)PO₄ ions, and can be formed by partially neutralizing phosphoric acid with lithium hydroxide prior to being combined with the rest of the precursor materials.

Suitable manganese precursors include manganese (II) compounds such as, for example, manganese (II) phosphate, manganese (II) hydrogen phosphate, manganese (II) dihydrogen phosphate, manganese (II) carbonate, manganese (II) hydrogen carbonate, manganese (II) formate, manganese (II) acetate, manganese (II) oxide, manganese (II) glycolate, manganese (II) lactate, manganese (II) citrate and manganese (II) tartrate. Manganese (II) hydrogen phosphate and manganese (II) phosphate will also function as all or part of the precursor for H_(x)PO₄ ions, and can be formed by partially neutralizing phosphoric acid with manganese metal prior to being combined with the rest of the precursor materials.

Suitable iron precursors include iron (II) phosphate, iron (II) hydrogen phosphate, iron (II) dihydrogen phosphate, iron (II) carbonate, iron (II) hydrogen carbonate, iron (II) formate, iron (II) acetate, iron (II) oxide, iron (II) glycolate, iron (II) lactate, iron (II) citrate, iron (II) tartrate. Iron (II) hydrogen phosphate and iron (II) phosphate will also function as all or part of the precursor for H_(z)PO₄ ions, and can be formed by partially neutralizing phosphoric acid with iron metal prior to being combined with the rest of the precursor materials.

Suitable precursors for the dopant metal include, for example, phosphate, hydrogen phosphate, dihydrogen phosphate, carbonate, formate, acetate, glycolate, lactate, tartrate, oxalate, oxide, hydroxide, fluoride, chloride, nitrate, sulfate, bromide and like salts of the various dopant metals. Examples include, for example, magnesium sulfate, magnesium phosphate, magnesium hydrogen phosphate, magnesium dihydrogen phosphate, magnesium carbonate, magnesium formate, magnesium acetate, cobalt (II) sulfate, cobalt (II) phosphate, cobalt (II) hydrogen phosphate, cobalt (II) dihydrogen phosphate, cobalt (II) carbonate, cobalt (II) formate, cobalt (II) acetate. The phosphates, hydrogen phosphates and dihydrogen phosphates in the foregoing list will in addition to serving as a source of the dopant metal ion also will serve as some or all of the source of H_(x)PO₄ ions.

Suitable precursors for H_(x)PO₄ ions include, in addition to the hydrogen phosphate and dihydrogen phosphate compounds listed above, phosphoric acid, tetraalkyl ammonium phosphate compounds, tetraphenyl ammonium phosphate compounds, ammonium phosphate, ammonium dihydrogen phosphate, and the like.

A carbonaceous material or precursor thereof may be included in the mixture that is taken to the milling step. Suitable carbonaceous materials and precursors include, for example, graphite, carbon black and/or other conductive carbon, various organic polymers, sugars such as sucrose or glucose, and the like.

In the milling process of the invention, the precursors typically are mixed in the presence of water or water/solvent mixture to form a slurry or thick paste. The precursors are typically added as solids, except that phosphoric acid (if used as the source of H_(x)PO₄ ions) is typically added as a concentrated aqueous solution. The solids content of this initial mixture may be, for example, from 25 to 60 weight percent. If the precursors react to form gaseous reaction by-products, it is generally preferred to permit the slurry to stand for a period to allow the gaseous by-products to liberate. Gaseous by-products often form, for example, when carbonate and/or carboxylate precursors are used. In a preferred process, the manganese, iron and dopant metal precursors are slurried with phosphoric acid, gaseous by-products are liberated, and then lithium hydroxide is added. The lithium hydroxide addition typically results in an exothermic reaction and a thickening of the slurry. It is believed that a mixture of intermediate reaction products such as MnHPO₄, Li₃PO₄, Li₂HPO₄, LiH₂PO₄, hurealite (Mn²⁺ ₅(PO₃OH)₂(PO4)₂.4H₂O), and Fe(C₂O₄) may form at this stage.

The milling step can be carried out in a wet or dry agitated media mill, such as a sand mill, ball mill, attrition mill, mechanofusion mill, or colloid mill. The mill is preferably a recirculating type, necessitating a carrier liquid which is preferably water. The milling can be performed in one or more steps to final particle size. In a two-step milling process, larger size grinding media such as 0.8-1.0 mm ceramic beads are initially used on the raw materials to provide a finely milled product having a particle size in the range of, for example, 0.2 to 1 microns. In the second step smaller grinding media such as 0.05 mm ceramic beads are used to further reduce the particle size into the range of, for example, 0.01 to 0.1 microns. In a single-step milling process, the raw materials are reduced in size to a product on the order of 0.01 to 0.1 micron in a single step.

The amount of milling energy used to mill the particles to the aforementioned particle sizes is typically 10 to 10,000 kW-h/ton (9.09 to 9091 kW-h/1000 kg) and preferably <500 kW-h/ton (<454.5 kW-h/1000 kg). An advantage of this invention that is seen in cases in which [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)]≠(3.0×Moles H_(x)PO₄) (such as from (2.95 to 2.98×Moles H_(x)PO₄) or (3.01 to 3.05×Moles H_(x)PO₄)) is that the amount of milling energy needed to achieve a product having good electrochemical performance is significantly lower (on the order of one-half) than that needed when [moles Li+(2×moles Mn)+(2×moles Fe)+(2×moles Dopant Metal)]=(3.0×moles H_(x)PO₄). This allows shorter milling times, and/or less expensive larger (0.3-1.2 mm diameter) milling media to be used with good results.

The milled product is calcined. Preferably, the milled materials are first suspended in a liquid diluent such as water and then spray dried to form secondary agglomerates of the primary milled particles. Each droplet sprayed into the drying chamber forms an aggregate of particles when dried. The size of this aggregate is related to the size of the droplet formed by the spray dryer atomizer. A suitable secondary aggregate size range is, for example, 2 to 25 microns (d50, by light scattering methods). These secondary aggregates (or the powders as received from the milling step) are then calcined to form the olivine phase. A suitable calcining temperature is 350 to 750° C. and preferably 500 to 700° C., for 0.1 to 20 hours and preferably 1 to 4 hours. Conditions are selected to avoid sintering the particles.

If a carbonaceous material or precursor thereof is present during the calcining step, the calcined particles will take the form of a nanocomposite of the olivine material and the carbonaceous material. The carbonaceous material may form a carbonaceous coating on the powdered particles, and/or form a layered composite therewith.

The cathode material in certain embodiments of the invention has the empirical formula Li_(a)Mn_(b)Fe_(c)D_(d)PO₄, based on the raw materials provided into the process. In this formula, a is a number from 0.85 to 1.15, preferably from 0.95 to 1.10, more preferably 1.0 to 1.1, still more preferably from 1.025 to 1.10 and in some embodiments 1.025 to 1.075. b is from 0.65 to 0.95, preferably 0.7 to 0.95, more preferably 0.70 to 0.85 and in some embodiments may be 0.75 to 0.85. c is from 0.049 to 0.349, preferably from 0.10 to 0.30 and more preferably 0.10 to 0.25. d is from 0.001 to 0.1, preferably from 0.005 to 0.1, more preferably 0.01 to 0.075 and in some embodiments may be 0.02 to 0.06. The values of a, b, c and d are such that 2.75≦(a+2b+2c+dV)≦3.05. Preferably (a+2b+2c+dV)≠3.00. In some embodiments, (a+2b+2c+dV) is 2.85 to 2.99 or 3.01 to 3.05. In particular embodiments, (a+2b+2c+dV) is 2.95 to 2.98 or 3.01 to 3.05.

At least a portion of the cathode material has an olivine structure. The olivine structure is revealed using X-ray diffraction methods. It is preferred that at least 50%, more preferably at least 80% and still more preferably at least 95% of the weight of the electroactive material is a Li_(a)Mn_(b)Fe_(c)D_(d)PO₄ having an olivine structure.

The cathode material preferably is formed during the milling and calcining processes into particles that may have flake-like, rod-like, spherical, or other morphologies and preferably have primary particle sizes of 200 nm or below. Primary particles sizes are conveniently measured by scanning electron microscopy. Aggregate sizes are conveniently measured by light scattering methods.

The lithium transition metal olivine produced in the process is useful as an electrode material, particularly as a cathode material, in various types of lithium batteries, especially secondary (rechargeable) batteries. It can be formulated into electrodes in any convenient manner, typically by blending it with a binder, forming a slurry and casting it onto a current collector. The electrode may contain particles and/or fibers of an electroconductive material such as graphite, carbon black, carbon fibers, carbon nanotubes, metals and the like. The lithium transition metal olivine particles may be formed into a nanocomposite with graphite, carbon black and/or other conductive carbon using, for example, dry ball milling processes as described in WO 2009/127901 and/or a wet milling process. Such a nanocomposite preferably contains at least 70% by weight of the lithium transition metal olivine particles, more preferably at least 75% by weight thereof, and up to 30%, more preferably 10 to 25%, by weight of carbon.

Secondary batteries containing a cathode which includes electroactive material of the invention exhibit unexpectedly good capacity retention upon battery cycling (i.e., subjecting the battery to repeated charge/discharge cycles), while retaining specific capacity and rate performance. In a secondary (rechargeable) battery, the good capacity retention correlates to long battery life and more consistent performance of the battery as it is repeatedly charged and discharged. This good capacity retention is seen at ambient temperature (20-25° C.) and at somewhat elevated temperatures (40-50° C.) as are often produced during the operation of an electrical device that contains the battery (and to which energy is supplied by the battery).

Cycle life testing for purposes of this invention is done using full cells with graphite anodes in pouches. The anodes, cathodes and separators are cut. Current tabs are welded onto the electrode sheet, the separator is sandwiched between the electrode sheets, and the sandwiched layers are sealed under vacuum in an aluminum laminated film bag filled with electrolyte (1.15 M LiPF₆ in ethylene carbonate/ethylmethylcarbonate (1:3 volume ratio)+2% vinylidene carbonate). The anode/cathode capacity ratio is kept between 1.1-1.2 to prevent lithium plating on to the anode and for good cycling performance. After sealing and assembly, the cells undergo the following formation cycle: 1^(st) cycle: C/20 to 4.2 V, C/100 cutoff (charge); rest 15 minutes at RT; C/20 to 2.7 V (discharge), 2^(nd) cycle: C/20-4.2 V, C/100 cutoff (charge); rest at 50° C. for 24 hours, and then degas at room temperature; C/20 to 2.7 V (discharge). After the formation cycle, the cells are cycled on a Maccor electrochemical tester at 50° C. under the following protocol: 1 C to 4.2 C, C/20 cutoff (charge), rest 15 minutes; C/10 to 2.7 V (discharge).

Lithium batteries containing cathodes containing the electroactive material of the invention also exhibit good specific capacity, which is often close to theoretical. Specific capacity is measured using half-cells at 25° C. on electrochemical testing using a Maccor 4000 electrochemical tester or equivalent electrochemical tester, using in order discharge rates of C/10, 1 C, 5 C, 10 C and finally C/10. The lithium transition metal olivine produced in accordance with the invention may have a specific capacity of at least 80%, at least 90% or even at least 93% of the theoretical capacity at 5 C testing in coin cells. For example, a lithium transition metal olivine made in accordance with the invention may exhibit, for example, a specific capacity of at least 140 mAh/g, at least 150 mAh/g, at least 155 mAh/g or even at least 160 mAh/g at a C/10 discharge rate, which values are close to the theoretical value of approximately 170 mAh/g.

A lithium battery containing such a cathode can have any suitable design. Such a battery typically comprises, in addition to the cathode, an anode, a separator disposed between the anode and cathode, and an electrolyte solution in contact with the anode and cathode. The electrolyte solution includes a solvent and a lithium salt.

Suitable anode materials include, for example, carbonaceous materials such as natural or artificial graphite, carbonized pitch, carbon fibers, graphitized mesophase microspheres, furnace black, acetylene black, and various other graphitized materials. Suitable carbonaceous anodes and methods for constructing same are described, for example, in U.S. Pat. No. 7,169,511. Other suitable anode materials include lithium metal, lithium alloys, other lithium compounds such as lithium titanate and metal oxides such as TiO₂, SnO₂ and SiO₂, as well as materials such as Si, Sn, or Sb.

The separator is conveniently a non-conductive material. It should not be reactive with or soluble in the electrolyte solution or any of the components of the electrolyte solution under operating conditions. Polymeric separators are generally suitable. Examples of suitable polymers for forming the separator include polyethylene, polypropylene, polybutene-1, poly-3-methylpentene, ethylene-propylene copolymers, polytetrafluoroethylene, polystyrene, polymethylmethacrylate, polydimethylsiloxane, polyethersulfones and the like.

The battery electrolyte solution has a lithium salt concentration of at least 0.1 moles/liter (0.1 M), preferably at least 0.5 moles/liter (0.5 M), more preferably at least 0.75 moles/liter (0.75 M), preferably up to 3 moles/liter (3.0 M), and more preferably up to 1.5 moles/liter (1.5 M). The lithium salt may be any that is suitable for battery use, including lithium salts such as LiAsF₆, LiPF₆, LiPF₄(C₂O₄), LiPF₂(C₂O₄)₂, LiBF₄, LiB(C₂O₄)₂, LiBF₂(C₂O₄), LiClO₄, LiBrO₄, LiIO₄, LiB(C₆H₅)₄, LiCH₃SO₃, LiN(SO₂C₂F₅)₂, and LiCF₃SO₃. The solvent in the battery electrolyte solution may be or include, for example, a cyclic alkylene carbonate like ethyl carbonate; a dialkyl carbonate such as diethyl carbonate, dimethyl carbonate or methylethyl carbonate, various alkyl ethers; various cyclic esters; various mononitriles; dinitriles such as glutaronitrile; symmetric or asymmetric sulfones, as well as derivatives thereof; various sulfolanes, various organic esters and ether esters having up to 12 carbon atoms, and the like.

The battery is preferably a secondary (rechargeable) battery, more preferably a secondary lithium battery. In such a battery, the charge reaction includes a dissolution or delithiation of lithium ions from the cathode into the electrolyte solution and concurrent incorporation of lithium ions into the anode. The discharging reaction, conversely, includes an incorporation of lithium ions into the cathode from the anode via the electrolyte solution.

The battery containing a cathode which includes lithium transition metal olivine particles made in accordance with the invention can be used in industrial applications such as electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, aerospace vehicles and equipment, e-bikes, etc. The battery of the invention is also useful for operating a large number of electrical and electronic devices, such as computers, cameras, video cameras, cell phones, PDAs, MP3 and other music players, tools, televisions, toys, video game players, household appliances, medical devices such as pacemakers and defibrillators, among many others.

The following examples are provided to illustrate the invention, but are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated.

Examples 1-3 and Comparative Samples A-D

Examples 1-3 and Comparative Samples A-D are prepared using a solid state process as described in WO 2009/144600.

TABLE 1 Designation Formula a + 2b + 2c + dV Comp. Sample A LiMn_(0.8)Fe_(0.2)PO₄ 3.0 Comp. Sample B Li_(1.025)Mn_(0.8)Fe_(0.2)PO₄ 3.025 Comp. Sample C Li_(1.1)Mn_(0.71)Fe_(0.24)PO₄ 3.0 Comp. Sample D Li_(1.1)Mn_(0.76)Fe_(0.19)PO₄ 3.0 Example 1 Li_(1.1)Mn_(0.8)Fe_(0.1)Mg_(0.05)PO₄ 3.0 Example 2 Li_(1.1)Mn_(0.8)Fe_(0.08)Mg_(0.07)PO₄ 3.0 Example 3 LiMn_(0.7)Fe_(0.2)Mg_(0.05)PO₄ 2.90

The resulting particles are mixed with vapor-grown carbon fibers and binder at a 93:2:5 weight ratio to form electrodes. The electrodes are given the same designations as the respective electroactive materials they contain (as indicated in Table 1 above).

Half cell specific capacities are measured at 20° C. at C/10 discharge rate. Newly made cells are cycled using a CC/CV charge protocol with 2 cycles at C/10 charge/discharge, followed by subsequent cycles at varying C-rates. Full cell cycle life is measured at elevated temperature (50° C.) at 1 C/1 C charge/discharge with CC/CV charging from 2.7 to 4.2V. Two formation cycles are done at room temperature (20° C.) at C/10 charge/discharge over the same voltage range prior to placing the cells at elevated temperature. Full cell cycle life results are indicated graphically in the FIGURE.

In the FIGURE, reference symbols A, B, C, D, 1, 2 and 3 represent the results obtained with the corresponding sample designation. The top portion of the FIGURE shows measured capacity (in mAh/g) plotted against the number of charge/discharge cycles. The bottom portion of the FIGURE shows the capacity retention, i.e., the percentage of the original capacity of each of the tested electrodes.

As can be seen from the FIGURE, Examples 1-3 are far superior to any of the Comparative Samples in retaining capacity over many charge/discharge cycles. Although these electrodes show a lower specific capacity initially, they retain much more of their capacity as the cell is cycled. Example 1 retains about 83% of its initial specific capacity, Example 2 retains 90% of its initial capacity and Example 3 retains only about 78% of its initial capacity. After 168 cycles, Examples 1-3 are equivalent even in absolute terms to the best of the Comparative Samples. Due to the superior capacity retention exhibited through the first 168 cycles Examples 1-3 are expected upon additional cycling to exhibit higher specific capacities in absolute terms than any of the Comparative Samples.

The electroactive material of Examples 1 and 2 both contain more than 1 atom of lithium per phosphate group (a>1) and are doped. Comparative Sample A is not doped and contains only 1 atom of lithium per phosphate group (a=1). The electrode made from that material retains little more than 60% of its initial capacity after only 140 cycles, and has a lower specific capacity at that point than either of Examples 1 or 2.

Comparative Samples B, C and D each has an excess of lithium (a>1) but are not doped. The electrode made using these material loses capacity quite rapidly upon charge/discharge cycling, and demonstrates that increasing the amount of lithium by itself provides little benefit.

Example 3 is doped with magnesium but contains only one atom of lithium per phosphate group (a=1). The data in Table 1 shows that doping by itself provides improved specific capacity retention in this test, but not as much as Examples 1 and 2.

Examples 4-8 and Comparative Sample E

Olivine LMFP particles having the formulae indicated in Table 2 below are made using the following process. In a mix tank with a high shear agitator (or rotor stator mixer), iron oxalate dihydrate (solid) and manganese carbonate (solid) are slurried with water to a concentration of 35-45 wt-% solids. In cases in which a dopant metal is included, the dopant metal precursor is magnesium acetate and/or cobalt acetate. 85% Phosphoric acid is slowly metered by a pump into the mix tank. Carbon dioxide becomes liberated as the phosphoric acid reacts with the manganese carbonate. After the acid addition is finished, the slurry is given about 30 minutes to mix to allow the carbon dioxide to continue to off gas. Then, lithium hydroxide monohydrate (solid) is added to the mix tank. The slurry goes through a viscous phase as the lithium hydroxide mixes with the solids. The LiOH addition is exothermic and the temperature rises to 55-60° C.

Once well mixed, the slurries are milled with a series of two mills. The materials are first loaded into a Centex disk style mill. The milling media size is approximately 0.7-0.9 mm in diameter. Once the mill is running at steady conditions, cellulose acetate powder is loaded. Aminopropanol and citric acid are added to control viscosity. The milling is continued until the particles are reduced to less than 40 microns. The material is then transferred into a Buhler Micromedia mill with milling media having a size of 0.1-0.3 mm in diameter. The milling is continued until the particles are reduced to approximately 50 nm in diameter.

The milled samples are spray dried at 170° C. to agglomerate the small particles into micron sized balls having diameters of up to about 20 microns. The spray dried particles contain approximately 5% water. The spray dried particles are calcined under an atmosphere containing <100 ppm oxygen. The particles are heated from room temperature to 400° C. over three hours and held at 400° C. for one hour. The temperature is then increased to 650° C. over two hours and held at 650° C. for three hours. CO, CO₂, H₂O, acetic acid and tars are given off throughout the calcination process. The particles are then cooled to below 60° C. and sieved through a 44 micron sieve.

TABLE 2 Designation Formula a + 2b + 2c + dV Ex. 4 Li_(1.05)Mn_(0.75)Fe_(0.1)Mg_(0.07)Co_(0.01)PO₄ 2.87 Ex. 5 Li_(1.05)Mn_(0.75)Fe_(0.24)Co_(0.01)PO₄ 3.05 Ex. 6 Li_(1.05)Mn_(0.7)Fe_(0.23)Mg_(0.03)Co_(0.01)PO₄ 2.99 Ex. 7 Li_(1.05)Mn_(0.75)Fe_(0.15)Mg_(0.04)Co_(0.01)PO₄ 2.95 Ex. 8 Li_(1.05)Mn_(0.71)Fe_(0.19)Mg_(0.04)Co_(0.01)PO₄ 2.95 Comp. Sample E Li_(1.05)Mn_(0.71)Fe_(0.24)PO₄ 2.95

Half-cell specific capacity at 25° C. is measured on each of electrode Examples 4-8 and for Comparative Sample A (Table 1) using a Maccor 4000 electrochemical tester. Testing is performed using half-cells using a charge/discharge rates 0.1 C, 5 C and 10 C. Results are as indicated in Table 3.

TABLE 3 Sample Specific Capacity (mAh/g) Designation 0.1 C 5 C 10 C Ex. 4 145 128 117 Ex. 5 155 135 108 Ex. 6 146 128 113 Ex. 7 148 120 100 Ex. 8 147 123 105 Comp. Sample E 154 135 118

Half and full cell battery for Examples 4-8 and Comparative Battery E are assembled. The anode in each case is lithium (in the case of a half cell) and a commercially available graphite (in the case of a full cell), the anode/cathode capacity ratio is 1.1 to 1.2 for the full cells, and the total cathode density is 1.4-1.6 g/cm³. A commercially available separator is used with an electrolyte of a 1.15 molar LiPF₆ solution in a 1:3 by weight mixture of ethylene carbonate and ethylmethylcarbonate that also contains 2% by weight vinylidene carbonate.

Discharge energies at 1/10 C, 1 C and 5 C discharge rates are measured at 25° C. on each of half cell (lithium anode) using 2025 coin cell configurations. The cells were charged using a constant current ( 1/10 C-rate) to 4.25V, and then held at constant voltage of 4.25 until the current decays to C/100. The discharge is done at constant current (varying with different c-rates) with a voltage cutoff of 2.7V. The discharge energy is calculated as the product of the integrated capacity vs. voltage. Results are as indicated in Table 4.

TABLE 4 Sample Discharge Energy (W-h/kg) Designation 0.1 C 1 C 5 C Ex. 4 577 535 470 Ex. 5 587 540 450 Ex. 6 570 530 450 Ex. 7 577 525 430 Ex. 8 570 525 440 Comp. Sample E 585 555 495

Full cell 50° C. cycling is performed on each of Battery Examples 4 through 8. Pouch cell configurations are used consisting of a single-side coated cathode and anode with a separator encased in a sealed pouch with electrolyte. Two formation cycles are done at room temperature, consisting of a constant current charge at 1/10 C-rate to 4.2V, with a constant voltage hold at 4.2V until the current decays to C/100. After a 5 minute rest, the cell is discharged at constant current (C/10) to a voltage cutoff of 2.7V. After the two initial cycles, the cell is moved to a 50° C. oven for cycling. The cycle test is done using a 1 C constant current charge to 4.2V, with a constant voltage hold at 4.2V until the current decays to C/100. The discharge is done at 1 C constant current with a 2.7V cutoff. Degassing of the pouch cell is done after 5 cycles at elevated temperature, by removal of the gases in the pouch and resealing of the pouch. Examples 4, 6, 7 and 8 all retain at least 80% of their initial capacity for 325 to 350 cycles at 50° C., 1 C/1 C cycling. Example 5 retains about 92.5% of its initial capacity only up to 250 cycles. These results, together with the results reported in Table 4, demonstrate the advantage of selecting the value of a+2b+2c+dV slightly below 3.00. Example 5, which has both excess lithium (a>1.0) and is Co doped but has an a+2b+2c+dV of 3.05, shows a greater loss of capacity at higher discharge rates and a faster loss of capacity on cycling than do the other examples, in which a+2b+2c+dV<3 and has both Mg and Co doping. Comparative Example E, which does not have any doping, exhibits poor cycle life, and can retain 80% of its capacity for fewer than 250 cycles.

Examples 9 and 10 and Comparative Samples E and F

Olivine LMFP particles having the formulae indicated in Table 5 below are made using the process described for Examples 1-3.

TABLE 5 Designation Formula a + 2b + 2c + dV Ex. 9 Li_(1.05)Mn_(0.75)Fe_(0.2)Mg_(0.02)Co_(0.01)PO₄ 3.01 Ex. 10 Li_(1.05)Mn_(0.75)Fe_(0.15)Mg_(0.05)Co_(0.01)PO₄ 2.97 Comp. Sample E Li_(1.05)Mn_(0.71)Fe_(0.24)PO₄ 2.95 Comp. Sample F Li_(1.0)Mn_(0.75)Fe_(0.25)PO₄ 3.00

Half-cell specific capacity at 25° C. and discharge energy are measured for each of electrode Examples 9 and 10 and for Comparative Samples E and F as described before Results are as indicated in Tables 6 and 7.

TABLE 6 Sample Specific Capacity (mAh/g) Designation C/10 1 C 5 C 10 C Ex. 9 148 140 124 112 Ex. 10 150 144 132 116 Comp. Sample E 147 139 118 93 Comp. Sample F 146 127 88 47

As can be seen from the data in Table 6, specific capacities are much higher for Examples 9 and 10 at higher discharge rates.

TABLE 7 C/10 V_(ave) discharge V_(ave) Sample CC/ (discharge), energy (discharge), Designation (CC + CV) C/10 (Wh/kg) 5 C Ex. 9 0.68 3.83 564 3.61 Ex. 10 0.71 3.86 579 3.54 Comp. Sample E 0.70 3.85 565 3.45 Comp. Sample F 0.38 3.80 553 3.29

As seen from the data in Table 7, all four of the tested electrode materials provide similar average voltage and discharge energy at a low (C/10) discharge rate. However, at a high (5 C) discharge rate, average operating voltage is significantly higher for Examples 9 and 10 than for the Comparatives. 

The invention claimed is:
 1. A particulate cathode material comprising an electroactive material having the empirical formula Li_(a)Mn_(b)Fe_(c)D_(d)PO₄, wherein a is a number from 1.025 to 1.10; b is from 0.70 to 0.85; c is from 0.1 to 0.3; d is from 0.01 to 0.075; (a+2b+2c+dV) is 2.85 to 2.99, wherein V is the valence of D, and D is a metal ion selected from one or more of magnesium, calcium, strontium, cobalt, titanium, zirconium, molybdenum, vanadium, niobium, nickel, scandium, chromium, copper, zinc, beryllium, lanthanum and aluminum, and further wherein at least a portion of the electroactive material has an olivine structure.
 2. The cathode material of claim 1 wherein D is magnesium, cobalt or a mixture of magnesium and cobalt.
 3. The cathode material of claim 2, wherein 1) a is 1.025 to 1.075, b is 0.70 to 0.85; c is 0.15 to 0.25, d is 0.02 to 0.06 and (a+2b+2c+dV) is 2.95 to 2.98. 